Conventionally, a vehicle steering apparatus having a rack-and-pinion steering gear unit has been widely used. FIGS. 16 to 20 illustrate an example of the conventional structure of such a steering apparatus. This steering apparatus has, as the general structure is shown in FIG. 16, a structure where a desired rudder angle can be provided to a non-illustrated right and left steering wheels by such a manner that the rotary movement of a steering wheel 1 operated by the driver is converted to a linear movement by a rack-and-pinion steering gear unit 5. To realize such a structure, specifically, the steering wheel 1 is fixed to the rear end portion of a steering shaft 2. Together with this, the front end portion of this steering shaft 2 is connected to the base end portion of a pinion shaft 6 included in the steering gear unit 5 through a pair of universal joints 3 and 3 and an intermediate shaft 4. Further, to both end portions of a rack shaft 7 included in this steering gear unit 5 and meshed with the pinion shaft 6, the base end portions of a pair of tie rods 8, 8 coupled to the right and left steering wheels, respectively, are connected.
The steering gear unit 5 has, as shown in detail in FIGS. 17 to 20, a housing 9, the pinion shaft 6, the rack shaft 7 and pressing means 10. The housing 9 is fixed to the vehicle body, and integrally includes a first housing portion 11 accommodating the intermediate portion of the rack shaft 7, a second housing portion 12 accommodating the front half portion of the pinion shaft 6 and a third housing portion 13 accommodating the pressing means 10. The pinion shaft 6 has pinion teeth 14 on a part on the side near the front end of the outer peripheral surface. Such a pinion shaft 6 is supported, in a state in which the front half portion is inserted in the second housing portion 12, so as to be only rotatable by a pair of rolling bearings 15 and 16 with respect to this second housing portion 12.
The rack shaft 7 has rack teeth 17 on a portion of a front face thereof near one end in the axial direction. The outer peripheral surface of this rack shaft 7 is a cylindrical surface except for the part where these rack teeth 17 are formed. That is, the cross-sectional shape of the outer peripheral surface of this rack shaft 7 is circular in the part other than the rack teeth 17 with respect to the axial direction, and in the part where these rack teeth 17 are formed with respect to the axial direction, the cross-sectional shape is linear in the part corresponding to these rack teeth 17 and arc-shaped in the remaining part. Such a rack shaft 7 is supported so as to be displaceable in the axial direction through a pair of guide bushes 18, 18 with respect to the first housing portion 11 in a state in which the intermediate part in the axial direction is inserted in the first housing portion 11 and the rack teeth 17 are meshed with the pinion teeth 14.
The guide bushes 18, 18 are formed of a low friction material such as a synthetic resin having oil resistance, a metal having a self-lubricating property or an oil-bearing metal so as to be cylindrical as a whole. The inner peripheral surfaces of the guide bushes 18, 18 have guiding protrusions 20, 20 in a plurality of positions in FIG. 18 in the circumferential direction, and the outer peripheral surfaces of the guide bushes 18, 18 have engagement protrusions 21, 21 in at least one position in FIG. 18 in the circumferential direction. FIG. 18 and FIG. 19 show an example disclosed in Patent Document 1. In this example, the inner peripheral surfaces of the guide bushes 18, 18 have the guiding protrusions 20, 20 in three positions substantially at regular intervals in the circumferential direction, and the outer peripheral surfaces of the guide bushes 18, 18 have the engagement protrusions 21, 21 in two positions at regular intervals in the circumferential direction. By engaging the engagement protrusions 21, 21 with engagement recesses 19, 19 formed on parts near the sides of both ends of the inner peripheral surface of the first housing portion 11, the guide bushes 18, 18 are fixed in a state of being fitted in fit-and-hold portions 36, 36 provided on parts near the sides of both ends of the inner peripheral surface of the first housing portion 11 in a state in which positioning in the circumferential direction is performed. In this state, distal end faces of the guiding protrusions 20, 20 are made to contact a portion of the outer peripheral of the rack shaft 7 other than the rack teeth 17 so as to be slidable in the axial direction.
The pressing means 10 is accommodated inside the third housing portion 13, and includes a pressing member 22 and a spring 23. The pressing surface, i.e., the distal end surface of the pressing member 22 is made to contact a portion of the back face of the rack shaft 7 on the side opposite to the pinion shaft 6 across this rack shaft 7 so that this rack shaft 7 is slidable in the axial direction. In this state, the pressing member 22 is elastically pressed toward the back face of the rack shaft 7 by the spring 23. Thereby, by providing a pre-load to the portion of meshing of the pinion teeth 14 and the rack teeth 17, the occurrence of an unusual noise at this meshing portion is suppressed and the operational feeling of the steering apparatus is improved. The pressing member 22 is made of a low friction material as a whole as mentioned above or has a low friction material layer on the pressing surface that is slidingly in contact with the back face of the rack shaft 7.
A front end portion of the intermediate shaft 4 is connected to a base end portion of the pinion shaft 6 of the steering gear unit 5 configured as described above via the universal joint 3. Together with this, to both end portions in the axial direction of the rack shaft 7, the base end portions of the tie rods 8, 8 are connected through ball joints 24, 24. The ball joints 24, 24 are fixed to both end portions of the rack shaft 7 by screwing or the like, respectively.
When the driver operates the steering wheel 1, this rotation of the steering wheel 1 is transmitted to the pinion shaft 6 through the steering shaft 2, the universal joints 3 and 3 and the intermediate shaft 4. Consequently, the rack shaft 7 is displaced in the axial direction, and together with this, the tie rods 8, 8 are pushed and pulled, whereby a desired rudder angle is provided to the right and left steering wheels.
However, in the case of the steering apparatus of the conventional structure as described above, there is a possibility that the axial displacement of the rack shaft 7 involved in the operation of the steering wheel 1 cannot be performed smoothly. That is, the roundness of the inner peripheral surfaces of the fit-and-hold portions 36, 36 provided on parts near the sides of both ends of the first housing portion 11 of the housing 9 is not always excellent. For this reason, if the guide bushes 18, 18 made of a material softer than the metal material of which the housing 9 is made are fixed in the state of being fitted (pushed) in the fit-and-hold portions 36, 36, there is a possibility that the guide bushes 18, 18 are elastically deformed to decrease the roundness of the inscribed circle of the guiding protrusions 20, 20 provided on the inner peripheral surfaces of the guide bushes 18, 18. Consequently, the contact pressure that acts on the portion where the apical surfaces of the guiding protrusions 20, 20 and the outer peripheral surface of the rack shaft 7 are slidingly in contact with each other becomes nonuniform, so that there is a possibility that the axial displacement of this rack shaft 7 cannot be performed smoothly.
On the contrary, Patent Document 2 and Patent Document 3 disclose guide bush structures capable of smoothly performing the axial displacement of the rack shaft passing inside even when the roundness of the inner peripheral surfaces of the fit-and-hold portions is insufficient. FIG. 21 and FIG. 22 show the structure of a guide bush disclosed in Patent Document 3. A guide bush 18a has a cylindrical fitting portion 25, a pair of elastic rings 26, 26, and a flange portion 27. The cylindrical fitting portion 25 is cylindrical as a whole, and is formed with slits 28, 28 at a plurality of locations along the circumferential direction in the axially inner end portion or the axially intermediate portion and slits 29, 29 in the axially outer end portion or the axially intermediate portion between the slits 28, 28 that are next to each other in the circumferential direction, so that the radial dimension can be expanded or reduced based on the presence of the slits 28, 29. Throughout the present description, the “inside” with respect to the axial direction is the central side in the direction of width of the vehicle body in a state of being incorporated in the vehicle and the left sides of FIGS. 1, 3 to 6, 8, 10, 13, 20 and 21. In contrast, the “outside” with respect to the axial direction is the outer side in the direction of width of the vehicle body in a state of being incorporated in the vehicle and the right sides of FIGS. 1, 3 to 6, 8, 10, 13, 20 and 21. On the cylindrical fitting portion 25, in order to adjust the pressure (secure ventilation) inside a housing 9a (first housing portion 11a), at the inner end portion in the axial direction thereof, a small diameter portion 37 is provided that is smaller in outside diameter than the intermediate portion or the outer end portion in the axial direction, and on the inner peripheral surface thereof, ventilating grooves 30a, 30b whose axial end portions are opened at the end faces of the cylindrical fitting portion 25 and the inner edges of the slits 28, 29, respectively, are provided. The elastic rings 26, 26 are fitted along catching grooves 31, 31 provided on the outer peripheral surface of the cylindrical fitting portion 25. The elastic rings 26, 26 are, like O rings, circular in cross section in free state, and the outside diameter (wire diameter) of the cross-sectional shape is greater than the depth of the catching grooves 31, 31 in free state. The flange portion 27 is provided to protrude outward in the radial direction from the outer peripheral surface of the axially outer end portion of the cylindrical fitting portion 25. In the case of the illustrated example, a catching groove 32 is provided along the entire circumference on the axially outer end surface of the flange portion 27, and an elastic ring 33 like an O ring is locked in the catching groove 32.
When attaching the guide bush 18a described above to a fit-and-hold portion 36a provided on a part near the sides of both ends in the axial direction of the first housing portion 11a of the housing 9a, the widths of the slits 28, 29 are reduced by using a cylindrical jig or the like, the cylindrical fitting portion 25 in a state of being reduced in diameter is inserted in this fit-and-hold portion 36a, the rack shaft 7 is then inserted into this cylindrical fitting portion 25 and this cylindrical fitting portion 25 is elastically increased in diameter. Thereby, the elastic rings 26, 26 are elastically compressed (held) between the bottom surfaces of the catching grooves 31, 31 and the inner peripheral surface of the fit-and-hold portion 36a. Together with this, the flange portion 27 is engaged with an engagement groove 34 formed at the outer end portion (opening side end portion) in the axial direction of this fit-and-hold portion 36a, and the elastic ring 33 is elastically compressed between the bottom surface of the catching groove 32 and the inside surface in the axial direction of an inward flange portion 35 provided at the outer end edge (opening end edge) in the axial direction of the first housing portion 11a. Thereby, the guide bush 18a and thus the rack shaft 7 are prevented from rattling in the first housing portion 11a. 
According to the structure of Patent Document 3 as described above, since the inner peripheral surface of the cylindrical fitting portion 25 is elastically pushed against the outer peripheral surface of the rack shaft 7 with an appropriate magnitude of force by the elastic rings 26, 26, even when the roundness of the inner peripheral surface of the fit-and-hold portion 36a is not excellent, the rack shaft 7 can be supported so that it can be smoothly displaced in the axial direction by the guide bush 18a. However, it is necessary to provide the slits 28, 29 and the ventilating grooves 30a, 30b at a plurality of locations along the circumferential direction of the cylindrical fitting portion 25, so that the abrasion resistance of the inner peripheral surface of this cylindrical fitting portion 25 may not be sufficiently ensured due to an increase of manufacturing cost of the guide bush 18a and a reduction of the abutting (sliding contact) area of the inner peripheral surface of the cylindrical fitting portion 25 and the outer peripheral surface of the rack shaft 7. Further, since the overall dimension in the axial direction of the guide bush 18a increases by providing the small diameter portion 37, problems such as cost rise occur. Further, since the guide bush 18a is largely reduced in diameter when attaching the guide bush 18a to the housing 9a, assembling work is deteriorated.